The invention relates to the lighting arts. It is especially applicable to roadway signal lighting such as pedestrian crosswalk signals (e.g., the ubiquitous walk/don't walk iconic lighted crosswalk signals found at most major urban intersections), and will be described with particular reference thereto. However, the invention will also find application in other lighting applications in which lighted icons, text, or other symbols are advantageously displayed with a wide viewing angle. The invention will additionally find application in general illumination, such as in desk lamps and illuminated magnifying glasses, where spatially and angularly uniform light output is desirable.
Lighted traffic signals are commonly used at busy traffic intersections. Such signs are visible day or night, and often include one or more symbols, such as letters, numerals, or symbolic icons, that are shown in distinctive colors. For example, the walk/don't walk pedestrian signs are typically dual signs that include a first sign indicating don't walk that shows an icon of a hand in the conventional stop position in the color red, as well as a second sign indicating walk that shows an icon of a walking person in white. Another example is the signs over traffic lanes that change direction between the morning and evening rush hours. These signs usually include a red “X” indicating an illegal lane direction, and a symbol in green indicating an acceptable direction of travel. Yet another example of traffic lighting is the conventional red, yellow, and green combination traffic control light. In this case the lights often do not display any symbols since the illuminated light color alone controls the traffic flow (i.e., green corresponds to go, yellow corresponds to caution, and “red” corresponds to “stop”). However, it is important that the traffic control light be viewable at wide viewing angles to maximize its visibility to automobile drivers and pedestrians.
Traffic signals should appear bright over a wide range of viewing angles. In the past, the use of light emitting diodes (LEDs) for such applications has been limited due to the high degree of directionality of the LED light source which restricts the effective viewing angle to angles relatively near to normal incidence. The prior art discloses use of an external lens to spread the LED light and increase the effective viewing angle. Because of the large amount of light refraction required to convert the highly directional LED light output to a more uniform beam output, conventional thick lenses are not appropriate. Instead, the prior art discloses using Fresnel lenses for this purpose. However, the use of Fresnel lenses for LED traffic signal lights has the disadvantage of typically reducing system efficiency by at least ten percent due to Fresnel losses in the lens.
The present invention contemplates an improved lighting apparatus that overcomes the above-mentioned limitations and others.